Megafires, also called “siege fires,” are the increasingly frequent blazes that burn 500,000 acres or more – 10 times the size of the average forest fire of 20 years ago. One of the current wildfires is the sixth biggest in California ever, in terms of acreage burned, according to state figures and news reports.The trend to more superhot fires, experts say, has been driven by a century-long policy of the US Forest Service to stop wildfires as quickly as possible. The unintentional consequence was to halt the natural eradication of underbrush, now the primary fuel for megafires.

Three other factors contribute to the trend, they add. First is climate change marked by a 1-degree F. rise in average yearly temperature across the West. Second is a fire season that on average is 78 days longer than in the late 1980s. Third is increased building of homes and other structures in wooded areas.

“We are increasingly building our homes … in fire-prone ecosystems,” says Dominik Kulakowski, adjunct professor of biology at Clark University Graduate School of Geography in Worcester, Mass. Doing that “in many of the forests of the Western US … is like building homes on the side of an active volcano.”

In California, where population growth has averaged more than 600,000 a year for at least a decade, housing has pushed into such areas.

“What once was open space is now residential homes providing fuel to make fires burn with greater intensity,” says Terry McHale of the California Department of Forestry firefighters union. “With so much dryness, so many communities to catch fire, so many fronts to fight, it becomes an almost incredible job.”